The present application relates to a light-transmissive optical sheet and a display device having such optical sheet incorporated therein.
Recently, liquid crystal display devices have been replacing cathode ray tubes (CRTs), the conventional main stream of display devices, by virtue of their advantages of low power consumption and space-saving property, possibility of cost-down and so forth. There are several types of liquid crystal display devices when classified typically by modes of illumination for image display, wherein representative ones include transmissive display device providing image display making use of a surface emission light source disposed behind a liquid crystal panel.
In such a transmissive display device, it is desirable to enhance the display luminance and to widen the angle of view, in view of raising the merchandise value. For this reason, a lens sheet and a reflective polarizer sheet having a diffuser function have been disposed between the surface emission light source and the liquid crystal panel, in this order as viewed from the surface emission light source (see Japanese Patent No. 3704364, Patent Document 1). By this configuration, diffusive light emitted from the surface emission light source is condensed by the lens sheet, and thereby the front luminance increases. The light thus condensed is separated by the reflective polarizer sheet into p- and s-waves, wherein only the p-wave is allowed to transmit therethrough. The s-wave herein is reflected by the reflective polarizer sheet, wherein the reflected s-wave is reflected again on a reflective sheet in the surface emission light source to be separated again into the p-wave and s-wave, and re-used. The light incident on the reflective polarizer sheet is diffused by the diffuser function of the reflective polarizer sheet, and widened in the angle of divergence. As a consequence, the display device is raised in the display luminance, and widened in the angle of view.
As described in the above, the lens sheet is used for enhancing the display luminance of the display device, but may be deflected due to heat of the surface emission light source towards the liquid crystal panel side, and such deflection may induce non-uniform luminance or may injure the liquid crystal panel. This problem becomes more distinctive as the screen size increases. Thickening of the lens sheet or composition thereof using a hard material may be possible measures for preventing the deflection. The thickening of the lens sheet will, however, thicken the display device to as much as such thickness. This is against the recent trend of thinning, and may degrade the merchandize value.